Optical waveguide element and optical modulation element

ABSTRACT

An optical waveguide element includes: an optical waveguide having a ridge part and a slab part having a thickness less than the ridge part. The optical waveguide includes: a first waveguide having a first ridge with and a first slab film thickness; a second waveguide having a second ridge width and a second slab film thickness; a first intermediate waveguide connected to the first waveguide and having a third ridge width and the first slab film thickness; and a second intermediate waveguide connected to the second waveguide and having a fourth ridge width and the second slab film thickness. The first waveguide, the first intermediate waveguide, the second intermediate waveguide, and the second waveguide are arranged in this order; the second slab film thickness is larger than the first slab film thickness; and the third ridge width is larger than the fourth ridge width.

TECHNICAL FIELD

The present invention relates to an optical waveguide element and an optical modulation element, more particularly, to a waveguide structure of a Mach-Zehnder optical modulation element.

BACKGROUND ART

Communication traffic has been remarkably increased with widespread Internet use, and optical fiber communication is becoming significantly important. The optical fiber communication is a technology that converts an electric signal into an optical signal and transmits the optical signal through an optical fiber and has wide bandwidth, low loss, and resistance to noise.

As a system for converting an electric signal into an optical signal, there are known a direct modulation system using a semiconductor laser and an external modulation system using an optical modulator. The direct modulation system does not require the optical modulator and is thus low in cost, but has a limitation in terms of high-speed modulation and, thus, the external modulation system is used for high-speed and long-distance applications.

The optical modulator is one of the typical electro-optical devices, and a Mach-Zehnder optical modulator in which an optical waveguide is formed by Ti (titanium) diffusion in the vicinity of the surface of a lithium niobate single-crystal substrate has been put to practical use (refer to, for example, a Patent Document 1). The Mach-Zehnder optical modulator uses an optical waveguide (Mach-Zehnder optical waveguide) having a Mach-Zehnder interferometer structure that separates light emitted from one light source into two beams, makes the two beams pass through different paths, and then recombines the two beams to cause interference. Although high-speed optical modulators having a modulation speed of 40 Gb/s or higher are commercially available, they have a major drawback that the entire length thereof is as long as about 10 cm.

On the other hand, a Patent Document 2 disclose a Mach-Zehnder optical modulator using a c-axis oriented lithium niobate film. The optical modulator using the lithium niobate film achieves significant reduction in size and driving voltage as compared with an optical modulator using the lithium niobate single-crystal substrate.

Patent Document 3 describes a ridge-shaped optical waveguide element having a waveguide layer formed of a lithium niobate film on a substrate, the waveguide layer including a slab part having a certain thickness and a ridge part protruding from the slab part. In this ridge-shaped optical waveguide element, the thickness of the slab part is less than 0.4 times the wavelength of light propagating through the ridge part, so that propagation loss can be reduced to a lower level even with a reduced ridge width. Further, as to an optical waveguide structure, Patent Document 4 describes that in order to connect an input/output waveguide part constituted by a ridge-shaped optical waveguide and an optical switch main part constitute by a high-mesa type optical waveguide, an input/output tapered waveguide part is provided between the input/output waveguide part and optical switch main part to change stepwise the shape of the waveguide.

CITATION LIST Patent Document

-   [Patent Document 1] Japanese Patent No. 4,485,218 -   [Patent Document 2] Japanese Patent Application Laid-open No.     2006-195383 -   [Patent Document 3] Japanese Patent Application Laid-open No.     2017-129834 -   [Patent Document 4] Japanese Patent No. 3,816,924

DISCLOSURE OF THE INVENTION Problem to Be Solved By the Invention

When a lithium niobate film epitaxially grown on a substrate is processed into a ridge shape to form an optical waveguide, light confinement effect can be enhanced by ensuring a sufficient height for a ridge part and reducing the film thickness of a slab part extending left and right from the ridge part. Thus, when voltage is applied between a pair of electrodes, a sufficient electric field can be applied to the optical waveguide, allowing a half-wavelength voltage Vπ to be reduced. The half-wavelength voltage Vπ is a difference (V₁ - V₂) between a voltage V₁ at which an optical output becomes maximum and a voltage V₂ at which the optical output becomes minimum, and a driving voltage is proportional to the half-wavelength voltage Vπ. Thus, a reduction in the half-wavelength voltage Vπ means a reduction in driving voltage.

However, when a DC bias is applied to such an optical waveguide, the lifetime of an optical modulation element is shortened due to large DC drift. The DC drift refers to the temporal change of an operating point of optical output. Typically, the optical output operating point is adjusted by a DC bias to an average value between the maximum optical output and minimum optical output; however, when the slab film thickness is reduced so as to ensure the height of the ridge part, the temporal change of operating point becomes large, disadvantageously resulting in a reduction in time (lifetime) to reach a state where the operating point cannot be adjusted even with application of a large DC bias.

An object of the present invention is therefore to provide an optical waveguide element capable of achieving a reduction in DC drift while reducing propagation loss of light and an optical modulation element using such an optical waveguide element. Another object of the present invention is to provide an optical modulation element capable of achieving a reduction both in driving voltage and in DC drift.

Means For Solving the Problems

As a result of intensive studies as to a structure of an optical modulation element that can reduce DC drift, the present inventors have obtained the following findings: DC drift depends on the slab film thickness of the ridge waveguide, and a reduction in the slab film thickness allows a reduction in driving voltage of the optical modulation element, whereas it causes an increase in DC drift. Further, the present inventors have found that when two optical waveguides having different slab film thicknesses are connected to each other, although connection loss increases due to a discontinuous variation in the slab film thickness, it can be reduced by adjusting the ridge widths of the two optical waveguides.

The present invention has been made based on the above findings, and an optical waveguide element according to the present invention includes: a substrate; and an optical waveguide formed of a thin film on the substrate and having a ridge part which is a protruding portion and a slab part having a film thickness smaller than that of the ridge part. The optical waveguide includes: a first waveguide part having a first ridge width (W₁) and a first slab film thickness (T_(sb1)); a second waveguide part having a second ridge width (W₂) and a second slab film thickness (T_(sb2)); a first intermediate waveguide part connected to the first waveguide part and having a third ridge width (W_(1c)) and the first slab film thickness (T_(sb1)); and a second intermediate waveguide part connected to the second waveguide part and having a fourth ridge width (W_(2c)) and the second slab film thickness (T_(sb2)). The first waveguide part, first intermediate waveguide part, second intermediate waveguide part, and second waveguide part are arranged in this order. The second slab film thickness (T_(sb2)) is larger than the first slab film thickness (T_(sb1)) (T_(sb2) > T_(sb1)). The third ridge width (W_(1c)) is larger than the fourth ridge width (W_(2c)) (W_(1c) > W_(2c)) .

According to the present invention, connection loss occurring due to connection between ridge waveguides having different slab film thicknesses can be reduced.

In the present invention, the third ridge width (W_(1c)) is preferably smaller than twice the fourth ridge width (W_(2c)) (W_(1c) < 2 x W_(2c)). This can prevent an increase in connection loss due to excessively large third ridge width (W_(1c)) of the first intermediate waveguide part as compared to the fourth ridge width (W_(2c)) of the second intermediate waveguide part.

In the present invention, the third ridge width (W_(1c)) is preferably larger than the first ridge width (W₁) (W_(1c) > W₁), and the fourth ridge width (W_(2c)) is preferably larger than the second ridge width (W₂) (W_(2c) > W₂) . This configuration is resistant to axis misalignment and can thus enhance the connection loss reduction effect.

In the present invention, the second ridge width (W₂) is preferably larger than the first ridge width (W₁) (W₂ > W₁). With this configuration, when the first and second waveguide parts are configured as an RF part and a DC part, respectively, it is possible to enhance DC drift reduction effect in the DC part while reducing driving voltage in the RF part.

In the preset invention, the optical waveguide preferably further includes a third intermediate waveguide part provided between the first and second intermediate waveguide parts and having a cross-sectional shape different from those of the first and second intermediate waveguide parts. In this case, the third intermediate waveguide part may have a third slab film thickness (T_(sb3)) smaller than the first slab film thickness (T_(sb1)), may have no slab part, and may have no waveguide shape. In either structure, it is possible to reduce connection loss occurring due to connection between the ridge waveguides having different slab film thicknesses.

In the present invention, a length (Lc) of the third intermediate waveguide part is preferably 3 µm or less (Lc ≤ 3 µm) . When the length of the third intermediate waveguide part is 3 µm or less, a waveguide structure small in connection loss can be achieved irrespective of the cross-sectional shape of the third intermediate waveguide part.

In the present invention, the first slab film thickness (T_(sb1)) is preferably less than 0.6 µm (0 µm ≤ T_(sb1) < 0.6 µm) , and the second slab film thickness (T_(sb2)) is preferably 0.6 µm or more (T_(sb2) ≥ 0.6 µm) . With this configuration, it is possible to achieve a reduction in DC drift in the DC part while reducing driving voltage in the RF part.

In the present invention, the thin film is preferably a lithium niobate film, and the c-axis of the lithium niobate film is preferably oriented perpendicular to the main surface of the substrate. With the ridge waveguide formed of such a lithium niobate film, propagation loss can be reduced to a lower level even with a reduced ridge width.

An optical modulation element according to the present invention includes: an optical waveguide element having the above features; a signal electrode that applies an RF signal to the first waveguide part; and a bias electrode that applies a DC bias to the second waveguide part. The thin film is made of an electrooptic material. According to the present invention, there can be provided an optical modulation element capable of reducing connection loss occurring due to connection between the ridge waveguides having different slab film thicknesses.

Advantageous Effects of the Invention

According to the present invention, there can be provided an optical waveguide element and an optical modulation element which are capable of achieving a reduction in DC drift while reducing propagation loss of light. Further, according to the present invention, there can be provided an optical modulation element capable of achieving a reduction both in driving voltage and in DC drift.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic plan views illustrating an optical modulation element according to a first embodiment of the present invention, in which FIG. 1A illustrates only an optical waveguide, and FIG. 1B illustrates the entire configuration of the optical modulating element including a traveling-wave electrode.

FIGS. 2A and 2B are schematic cross-sectional views of the optical modulation element, in which FIG. 2A is a cross-sectional view of the RF part taken along the line X₁-X₁' in FIGS. 1A and 1B, and FIG. 2B is a cross-sectional view of the DC part 3 b taken along the line X₂-X₂' in FIGS. 1A and 1B.

FIG. 3 includes a schematic plan view and schematic cross-sectional views illustrating a waveguide structure around the intermediate part between the RF part and DC part.

FIG. 4 is a schematic perspective view three-dimensionally illustrating the waveguide structure in the intermediate part.

FIG. 5 is a schematic plan view illustrating a case where an axis misalignment occurs between the optical waveguide on the RF part side and optical waveguide on the DC part side.

FIG. 6 shows an optical modulation element according to a second embodiment of the present invention, and includes a schematic plan view and schematic cross-sectional views illustrating a waveguide structure around the intermediate part between the RF part and DC part.

FIG. 7 is a schematic perspective view three-dimensionally illustrating the waveguide structure in the intermediate part illustrated in FIG. 6 .

FIGS. 8A to 8C are views for explaining a formation method for a waveguide structure having the third intermediate waveguide part illustrated in FIGS. 6 and 7 .

FIG. 9 shows an optical modulation element according to a third embodiment of the present invention, and includes a schematic plan view and schematic cross-sectional views illustrating a waveguide structure around the intermediate part between the RF part and DC part.

FIG. 10 is a schematic perspective view three-dimensionally illustrating the waveguide structure in the intermediate part 3 c illustrated in FIG. 9 .

FIGS. 11A to 11B are views for explaining a formation method for a waveguide structure having the third intermediate waveguide part illustrated in FIGS. 9 and 10 .

FIGS. 12A and 12B are schematic cross-sectional views of an optical modulation element according to a fourth embodiment of the present invention, in which FIG. 12A is a cross-sectional view of the RF part, and FIG. 12B is a cross-sectional view of the DC part.

FIG. 13 is a schematic plan view illustrating a waveguide structure around the intermediate part between the RF part and DC part.

FIG. 14 are schematic cross-sectional views of a structure of a DC part of an optical modulation element according to a fifth embodiment of the present invention.

FIG. 15 is a graph showing a relationship between the slab film thickness T_(sb1) of the ridge waveguide in the RF portion and the electric field efficiency VnL.

FIG. 16 is a graph showing the relationship between the ridge width W_(1c) of the first intermediate part on the RF part side and the connection loss (dB).

FIG. 17 is a graph showing a relationship between the magnitude ΔW of the axial deviation between the first intermediate waveguide part and the second intermediate waveguide part and the connection loss (dB).

FIG. 18 is a graph showing a relationship between the length L_(c) of the third intermediate waveguide part and the connection loss (dB).

MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

FIGS. 1A and 1B are schematic plan views illustrating an optical modulation element according to a first embodiment of the present invention. FIG. 1A illustrates only an optical waveguide, and FIG. 1B illustrates the entire configuration of the optical modulating element including a traveling-wave electrode.

As illustrated in FIGS. 1A and 1B, an optical modulation element 1 includes a Mach-Zehnder optical waveguide 2 formed on a substrate 10 and having first and second optical waveguides 2 a and 2 b provided in parallel to each other, a first signal electrode 4 a provided along the first waveguide 2 a, a second signal electrode 4 b provided along the second waveguide 2 b, a first bias electrode 5 a provided along the first waveguide 2 a, and a second bias electrode 5 b provided along the second waveguide 2 b. The first and second signal electrodes 4 a and 4 b constitute an RF interaction part 3 ir of the Mach-Zehnder optical modulation element together with the first and second waveguides 2 a and 2 b. The first and second bias electrodes 5 a and 5 b constitute a DC interaction part 3 id of the Mach-Zehnder optical modulation element together with the first and second waveguides 2 a and 2 b.

The Mach-Zehnder optical waveguide 2 is an optical waveguide element having a Mach-Zehnder interferometer structure. The Mach-Zehnder optical waveguide 2 has an input waveguide 2 i, a demultiplexer 2 c demultiplexing light propagating through the input waveguide 2 i, first and second waveguides 2 a and 2 b extending in parallel from the demultiplexer 2 c, a multiplexer 2 d multiplexing lights propagating through the first and second waveguides 2 a and 2 b, and an output waveguide 2 o through which light output from the multiplexer 2 d propagates. Light input to the input waveguide 2 i is demultiplexed at the demultiplexer 2 c. The demultiplexed lights travel through the first and second waveguides 2 a and 2 b, respectively, and multiplexed at the multiplexer 2 d. The multiplexed light is then output from the output waveguide 2 o as a modulated light.

The first and second signal electrodes 4 a and 4 b are linear electrode patterns overlapping the first and second waveguides 2 a and 2 b, respectively, in a plan view. Both ends of each of the first and second signal electrodes 4 a and 4 b are drawn to the vicinity of the outer peripheral end of the substrate 10. More specifically, one ends 4 a ₁ and 4 b ₁ of the first and second signal electrodes 4 a and 4 b are each drawn to the vicinity of the edge of the substrate 10 to constitute a signal input port, and a driver circuit 9 a is connected to the signal input ports. The other ends 4 a ₂ and 4 b ₂ of the first and second signal electrodes 4 a and 4 b are drawn to the vicinity of the edge of the substrate 10 and connected to each other through a terminal resistor 9 b. As a result, the first and second signal electrodes 4 a and 4 b function as a differential coplanar traveling-wave electrode.

The first and second bias electrodes 5 a and 5 b are provided independently of the first and second signal electrodes 4 a and 4 b, respectively, so as to apply a DC bias to the first and second waveguides 2 a and 2 b. One ends 5 a ₁ and 5 b ₁ of the first and second bias electrodes 5 a and 5 b are each drawn to the vicinity of the substrate 10 to constitute a DC bias input port, and a bias circuit 9 c is connected to the DC bias input ports. Although, in the present embodiment, the first and second bias electrodes 5 a and 5 b are positioned closer to the output terminal side of the Mach-Zehnder optical waveguide 2 than the first and second signal electrodes 4 a and 4 b, they may be positioned closer to the input terminal side.

Thus, the first and second signal electrodes 4 a and 4 b constitute an RF part 3 a that applies an RF signal to the first and second waveguides 2 a and 2 b, and first and second bias electrodes 5 a and 5 b constitute a DC part 3 b that applies a DC bias to the first and second waveguides 2 a and 2 b. An intermediate part 3 c between the RF part 3 a and DC part 3 b has neither the signal electrode nor the bias electrode. Although details will be described later, the optical waveguide in the intermediate part 3 c has a particular shape so as to connect the optical waveguide in the RF part 3 a and that in the DC part 3 b.

Differential signals (modulated signals) having the same absolute value but opposite polarities are input to the one ends of the first and second signal electrodes 4 a and 4 b. The first and second waveguides 2 a and 2 b are each formed of a material, such as lithium niobate, having electrooptic effect, so that the refractive indices of the first and second waveguides 2 a and 2 b are changed with +Δn and -Δn by an electric field applied to the first and second waveguides 2 a and 2 b, with the result that a phase difference between the pair of optical waveguides is changed. A signal light modulated by the change in the phase difference is output from the output waveguide 2 o.

As described above, the optical modulation element 1 according to the present embodiment is of a dual drive type constituted by the pair of signal electrodes, so that it is possible to increase the symmetry of an electric field to be applied to the pair of optical waveguides to thereby reduce a wavelength chirp.

FIGS. 2A and 2B are schematic cross-sectional views of the optical modulation element 1, in which FIG. 2A is a cross-sectional view of the RF part 3 a taken along the line X₁-X₁' in FIGS. 1A and 1B, and FIG. 2B is a cross-sectional view of the DC part 3 b taken along the line X₂-X₂' in FIGS. 1A and 1B.

As illustrated in FIGS. 2A and 2B, the optical modulation element 1 has a multilayer structure including a substrate 10, a waveguide layer 11, a protective layer 12, a buffer layer 13, and an electrode layer 14, which are laminated in this order.

The substrate 10 is, e.g., a sapphire substrate, and the waveguide layer 11 of an electrooptic material, such as lithium niobate, is formed on the main surface of the substrate 10. The waveguide layer 11 has ridge parts 11 r, i.e., protruding portions, and a slab part 11 s provided on both sides of each of the ridge part 11 r and each having a reduced thickness, and the ridge parts 11 r constitute the first and second waveguides 2 a and 2 b, respectively. A width W₁ (first ridge width) of the ridge part 11 r in the RF part 3 a and a width W₂ (second ridge width) of the ridge part 11 r in the DC part 3 b can be set to 0.5 µm to 5 µm. Although the width W₁ of the ridge part 11 r in the RF part 3 a and width W₂ of the ridge part 11 r in the DC part 3 b are equal to each other in the present embodiment, they may differ from each other.

The ridge part 11 r is a main part of the optical waveguide. As described above, the ridge part 11 r refers to an upwardly protruding portion. The protruding portion has a larger film thickness of the electrooptic material film than that of left and right portions thereof and thus has a high effective refractive index. This allows light to be confined also in the left-right direction, so that a function as a three-dimensional optical waveguide is achieved. The ridge part 11 r may be any shape as long as it can guide light and has a larger film thickness of the electrooptic material film than that of left and right portions thereof. Therefore, the ridge part 11 r may have an upwardly protruding dome shape or an upwardly protruding triangular shape. In forming the ridge part 11 r, a mask such as a resist is formed on the electrooptic material, and the electrooptic material is selectively etched for patterning. The width, height, shape, and the like of the ridge part 11 r need to be optimized so as to improve device characteristics.

The thickness of the ridge part 11 r is typically equal to the thickness of the electrooptic material film. The width (ridge widths W₁, W₂) of the ridge part 11 r is defined as the width of the upper surface of the ridge part 11 r. This is because the side surface of the ridge part 11 r may be inclined, although it is vertical to the substrate 10 in the illustrated example. The inclination angle of the side surface of the ridge part 11 r is preferably close to 90° and may be at least 70°. When the width of the upper surface of the ridge part is set as the ridge width, the ridge width can clearly be defined even when the ridge part 11 r has a trapezoidal shape.

The slab part 11 s provided on both sides of each of the ridge part 11 r is a part extending left and light from the ridge part 11 r and formed of an electrooptic material film having a thickness smaller than the electrooptic material film of the ridge part 11 r. In the present embodiment, the slab part 11 s has a substantially constant thickness; however, the slab part 11 s in the vicinity of the root of the ridge part 11 r may include a gentle tapered shape or a depression, so that the slab film thickness is unstable. Thus, the thickness of the slab part 11 s is defined not as the thickness of a part where the film thickness transiently varies but as the thickness of a part slightly apart from the root of the ridge part 11 r where the film thickness is stable.

A thickness T_(sb1) (first slab film thickness) of the slab part 11 s in the RF part 3 a and a thickness T_(sb2) (second slab film thickness) of the slab part 11 s in the DC part 3 b differ from each other. In the present embodiment, the thickness T_(sb2) of the slab part 11 s in the DC part 3 b is preferably larger than the thickness T_(sb1) of the slab part 11 s in the RF part 3 a. By thus increasing the thickness of the slab film thickness of the ridge wave guide in the DC part 3 b, DC drift can be reduced to prolong the lifetime of the optical modulation element. Further, by reducing the slab film thickness of the ridge waveguide in the RF part 3 a to increase the protruding height of the ridge part 11 r, light confinement effect can be enhanced to improve light modulation efficiency.

The protective layer 12 is formed in an area not overlapping the first and second waveguides 2 a and 2 b in a plan view. The protective layer 12 covers the entire area of the upper surface of the waveguide layer 11 excluding portions where the ridge parts 11 r are formed, and the side surfaces of each of the ridge parts 11 r are also covered with the protective layer 12, so that scattering loss caused due to the roughness of the side surfaces of the ridge part 11 r can be prevented. The thickness of the protective layer 12 is substantially equal to the height of the ridge part 11 r of the waveguide layer 11. There is no particular restriction on the material of the protective layer 12 and, for example, silicon oxide (SiO₂) may be used.

The buffer layer 13 is formed on at least the upper surfaces of the ridge parts 11 r so as to prevent light propagating through the first and second waveguides 2 a and 2 b from being absorbed by the first and second signal electrodes 4 a and 4 b. The buffer layer 13 is preferably formed of a material having a lower refractive index than those of the waveguide layer 11 and a high transparency, such as Al₂O₃, SiO₂, LaAlO_(s), LaYO₃, ZnO, HfO₂, MgO, or Y₂O₃. The thickness of the buffer layer 13 on the upper surface of the ridge part 11 r may be about 0.2 µm to 1 µm. The buffer layer 13 is more preferably formed of a material having a high dielectric constant. In the present embodiment, although the buffer layer 13 covers not only the upper surfaces of the respective first and second waveguides 2 a and 2 b, but also the entire underlying surface including the upper surface of the protective layer 12, it may be patterned so as to selectively cover only the vicinity of the upper surfaces of the first and second waveguides 2 a and 2 b. Further, the buffer layer 13 may be directly formed over the entire upper surface of the waveguide layer 11 with the protective layer 12 omitted.

The film thickness of the buffer layer 13 is preferably as large as possible in order to reduce light absorption by an electrode and preferably as small as possible in order to apply a high electric field to the optical waveguide. The light absorption and applied voltage of an electrode have a trade-off relation, so that it is necessary to set adequate film thickness according to the purpose. The dielectric constant of the buffer layer 13 is preferably as high as possible, because the higher the dielectric constant thereof, the more VπL (index representing electric field efficiency) is reduced. Further, the refractive index of the buffer layer 13 is preferably as low as possible, because the lower the refractive index thereof, the thinner the buffer layer 13 can be. In general, a material having a high dielectric constant has a higher refractive index, so that it is important to select a material having a high dielectric constant and a relatively lower refractive index considering the balance therebetween. For example, A1₂O₃ has a specific dielectric constant of about 9 and a refractive index of about 1.6 and is thus preferable. LaAlO_(s) has a specific dielectric constant of about 13 and a refractive index of about 1.7, and LaYO₃ has a specific dielectric constant of about 17 and a refractive index of about 1.7 and are thus particularly preferable.

The buffer layer 13 in the RF part 3 a illustrated in FIG. 2A and buffer layer 13 in the DC part 3 b illustrated in FIG. 2B may be constituted by different materials. A buffer layer material that can optimize the characteristics of the RF part 3 a is used for the buffer layer 13 in the RF part 3 a, and a buffer layer material that can reduce DC drift is used for the buffer layer 13 in the DC part 3 b, whereby the characteristics of the buffer layers 13 both in the RF part 3 a and DC part 3 b can be optimized. For example, as the material that can reduce DC drift, a material containing silicon oxide and indium oxide may be used.

As illustrated in FIG. 2A, the electrode layer 14 in the RF part 3 a is provided with the first and second signal electrodes 4 a and 4 b. The first signal electrode 4 a is provided overlapping the ridge part 11 r corresponding to the first waveguide 2 a so as to modulate light traveling inside the first waveguide 2 a and is opposed to the first waveguide 2 a through the buffer layer 13. The second signal electrode 4 b is provided overlapping the ridge part 11 r corresponding to the second waveguide 2 b so as to modulate light traveling inside the second waveguide 2 b and is opposed to the second waveguide 2 b through the buffer layer 13.

As illustrated in FIG. 2B, the electrode layer 14 in the DC part 3 b is provided with the first and second bias electrodes 5 a and 5 b. The first bias electrode 5 a is provided overlapping the ridge part 11 r corresponding to the first waveguide 2 a so as to provide electric field biasing of light traveling inside the first waveguide 2 a and is opposed to the first waveguide 2 a through the buffer layer 13. The second bias electrode 5 b is provided overlapping the ridge part 11 r corresponding to the second waveguide 2 b so as to provide electric field biasing of light traveling inside the second waveguide 2 b and is opposed to the second waveguide 2 b through the buffer layer 13.

As illustrated in FIGS. 2A and 2B, in the cross section perpendicular to the extending direction of first and second waveguides 2 a and 2 b, an electrode structure is left-right symmetric. Thus, the magnitudes of electric fields applied from the first and second signal electrodes 4 a and 4 b to the respective first and second waveguides 2 a and 2 b are made the same as much as possible, whereby the wavelength chirp can be reduced. In the present invention, the electrode structure is not particularly limited and may be of a so-called a single-drive type, and the presence/absence of a ground electrode and the layout thereof are not particularly limited.

The waveguide layer 11 is not particularly limited as long as it is made of an electrooptic material and is preferably made of lithium niobate (LiNbO₃). This is because lithium niobate has a large electrooptic constant and is thus suitable as the constituent material of an optical device such as an optical modulation element. An explanation will now be given of the configuration of the present embodiment, where the waveguide layer 11 is a lithium niobate film.

Although the substrate 10 is not particularly limited as long as it has a lower refractive index than that of a lithium niobate film, the substrate 10 is preferably a substrate on which the lithium niobate film can be formed as an epitaxial film. Specifically, the substrate 10 is preferably a sapphire single crystal substrate or a silicon single crystal substrate. The crystal orientation of the single crystal substrate is not particularly limited. The lithium niobate film can be easily formed as a c-axis oriented epitaxial film on single crystal substrates having different crystal orientations. Since the c-axis oriented lithium niobate film has three-fold symmetry, the underlying single crystal substrate preferably has the same symmetry. Thus, when the sapphire single crystal substrate is used as the substrate 10, it preferably has a c-plane, and when the silicon single crystal substrate is used as the substrate 10, it preferably has a (111) surface.

The epitaxial film refers to a film having the crystal orientation of the underlying substrate or film. When the film in-plane surface is defined as an X-Y plane, and the film thickness direction is as a Z-axis, the crystal is uniformly oriented along the X-, Y-, and Z-axes. For example, an epitaxial film can be confirmed by first measuring the peak intensity at the orientation position by 2θ-θ X-ray diffraction and secondly observing poles.

Specifically, first, in the 2θ-θ X-ray diffraction measurement, all the peak intensities except for the peak intensity on a target surface must be equal to or less than 10%, preferably equal to or less than 5%, of the maximum peak intensity on the target surface. For example, in a c-axis oriented epitaxial lithium niobate film, the peak intensities except for the peak intensity on a (00L) surface are equal to or less than 10%, preferably equal to or less than 5%, of the maximum peak intensity on the (00L) surface. (00L) is a general term for (001), (002), and other equivalent surfaces.

Secondly, poles must be observed in the measurement. Under the condition where the peak intensities are measured at the first orientation position, only the orientation in a single direction is proved. Even if the first condition is satisfied, in the case of nonuniformity in the in-plane crystalline orientation, the X-ray intensity is not increased at a particular angle, and poles cannot be observed. Since LiNbO₃ has a trigonal crystal system, single crystal LiNbO₃ (014) has three poles.

For the lithium niobate film, it is known that crystals rotated by 180° about the c-axis are epitaxially grown in a symmetrically-coupled twin crystal state. In this case, three poles are symmetrically-coupled to form six poles. When the lithium niobate film is formed on a single crystal silicon substrate having a (100) surface, the substrate has four-fold symmetry, and 4 x 3 = 12 poles are observed. In the present invention, the lithium niobate film epitaxially grown in the twin crystal state is also considered to be an epitaxial film.

The lithium niobate film has a composition of LixNbAyOz. A denotes an element other than Li, Nb, and O, wherein x ranges from 0.5 to 1.2, preferably 0.9 to 1.05, y ranges from 0 to 0.5, and z ranges from 1.5 to 4, preferably 2.5 to 3.5. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, alone or a combination of two or more of them.

The lithium niobate film preferably has a film thickness of 2 µm or less. This is because a high-quality lithium niobate film having a thickness more than 2 µm is difficult to form. The lithium niobate film having an excessively small thickness cannot completely confine light in it, disadvantageously allowing the light to penetrate through the substrate 10 and/or the buffer layer 13. Application of an electric field to the lithium niobate film may therefore cause a change in the effective refractive index of the optical waveguide (2 a, 2 b) to decrease. Thus, the lithium niobate film preferably has a film thickness that is at least approximately one-tenth of the wavelength of light to be used.

The lithium niobate film is preferably formed using a film formation method, such as sputtering, CVD or sol-gel process. Application of an electric field in parallel to the c-axis of the lithium niobate that is oriented perpendicular to the main surface of the substrate 10 can change the optical refractive index in proportion to the electric field. In the case of the single crystal substrate made of sapphire, the lithium niobate film can be directly epitaxially grown on the sapphire single crystal substrate. In the case of the single crystal substrate made of silicon, the lithium niobate film is epitaxially grown on a clad layer (not illustrated). The clad layer (not illustrated) has a refractive index lower than that of the lithium niobate film and should be suitable for epitaxial growth. For example, a high-quality lithium niobate film can be formed on a clad layer (not illustrated) made of Y₂O₃.

As a formation method for the lithium niobate film, there is known a method of thinly polishing or slicing the lithium niobate single crystal substrate. This method has an advantage that characteristics same as those of the single crystal can be obtained and can be applied to the present invention.

As illustrated in FIG. 2A, the width of the first and second signal electrodes 4 a and 4 b is slightly larger than the ridge width W₁ of the first and second waveguides 2 a and 2 b of a ridge-shaped lithium niobate film. To concentrate an electric field from the first and second signal electrodes 4 a and 4 b on the first and second waveguides 2 a and 2 b, the width of the first and second signal electrodes 4 a and 4 b is preferably 1.1 times to 15 times, more preferably, 1.5 times to 10 times the ridge width W₁ of the first and second waveguides 2 a and 2 b.

As illustrated in FIG. 2B, the width of the first and second bias electrodes 5 a and 5 b is slightly larger than the ridge width W₂ of the first and second waveguides 2 a and 2 b of a ridge-shaped lithium niobate film. To concentrate an electric field from the first and second bias electrodes 5 a and 5 b on the first and second waveguides 2 a and 2 b, the width of the first and second bias electrodes 5 a and 5 b is preferably 1.1 times to 15 times, more preferably, 1.5 times to 10 times the ridge width W₂ of the first and second waveguides 2 a and 2 b.

The slab part 11 s of the first and second waveguides 2 a and 2 b in the DC part 3 b illustrated in FIG. 2B is thicker than the slab part 11 s of the first and second waveguides 2 a and 2 b in the RF part 3 a illustrated in FIG. 2A. By thus increasing the slab film thickness T_(sb2) of the ridge wave guide in the DC part 3 b, DC drift can be reduced. Further, by reducing the slab film thickness T_(sb1) of the ridge waveguide in the RF part 3 a, effect of confining light propagating through the optical waveguide can be enhanced to improve light modulation efficiency, achieving a reduction in driving voltage as much as possible.

Although the reason that an increase in the slab film thickness in the DC part 3 b reduces DC drift is unclear, it is thought that damage occurring at a time of processing the lithium niobate film into a ridge shape has influence on DC drift. To reduce the slab film thickness (to increase the protruding height of the ridge part), it is necessary to more deeply dig the upper surface of the lithium niobate film, and thus a surface to be processed is significantly damaged. On the other hand, when the slab film thickness is increased (the protruding height of the ridge part is reduced), the amount of processing applied to the lithium niobate film is small, so that a surface to be processed is less damaged. For the above reason, it is considered that DC drift is reduced to prolong the lifetime of the DC part 3 b.

When the protruding height of the ridge part 11 r constituting the first and second waveguides 2 a and 2 b in both the RF part 3 a and DC part 3 b is relatively increased to reduce the thicknesses T_(sb1) and T_(sb2) (slab film thickness) of the slab part 11 s formed on both sides of the ridge part 11 r, effect of confining light propagating through the optical waveguide can be increased to achieve a reduction in driving voltage as much as possible. However, when such a waveguide structure is applied to the DC part, DC drift increases to fail to prolong the lifetime of the optical modulation element. On the other hand, when the protruding height of the ridge part 11 r in both the RF part 3 a and DC part 3 b is relatively reduced to increase the thicknesses T_(sb1) and T_(sb2) (slab film thickness) of the slab part 11 s, DC drift can be reduced, whereas the half-wavelength voltage V_(π) increases to fail a reduction in driving voltage.

However, in the present embodiment, the slab film thickness of the optical waveguide in the DC part 3 b is made different from that in the RF part 3 a such that the slab film thickness in the DC part 3 b is relatively increased and that the slab film thickness in the RF part 3 a is relatively reduced, whereby both the DC drift reduction effect in the DC part 3 b and reduction in driving voltage in the RF part 3 a can be achieved.

When the slab film thickness T_(sb1) of the first and second waveguides 2 a and 2 b in the RF part 3 a and the slab film thickness T_(sb1) of the first and second waveguides 2 a and 2 b in the DC part 3 b are different, simply connecting the waveguides in the RF part 3 a and DC part 3 b disadvantageously increases connection loss due to mismatch of the waveguide shape. Thus, in the present embodiment, an intermediate waveguide part is provided in the intermediate part 3 c between the RF part 3 a and DC part 3 b so as to enhance a level of matching between the waveguides in the RF part 3 a and DC part 3 b. Hereinafter, the configuration of the intermediate waveguide part will be described in detail.

FIG. 3 includes a schematic plan view and schematic cross-sectional views illustrating a waveguide structure around the intermediate part 3 c between the RF part 3 a and DC part 3 b. FIG. 4 is a schematic perspective view three-dimensionally illustrating the waveguide structure in the intermediate part 3 c.

As illustrated in FIGS. 3 and 4 , the first and second waveguides 2 a and 2 b each includes a first waveguide part 21 a which is an optical waveguide in the RF part 3 a, a second waveguide part 21 b which is an optical waveguide in the DC part 3 b, a first intermediate waveguide part 22 a provided in the intermediate part 3 c on the side close to the RF part 3 a so as to be connected to the first waveguide part 21 a, and a second intermediate waveguide part 22 b provided in the intermediate part 3 c on the side close to the DC part 3 b so as to be connected to the second waveguide part 21 b. That is, the first and second waveguides 2 a and 2 b each have a configuration in which the first waveguide part 21 a, first intermediate waveguide part 22 a, second intermediate waveguide part 22 b, and second waveguide part 21 b are arranged in this order.

The first waveguide part 21 a has the ridge width W₁ and slab film thickness T_(sb1), and second waveguide part 21 b has the ridge width W₂ (= W₁) and slab film thickness Tsb2 (> T_(sb1)). When the first and second waveguide parts 21 a and 21 b having different slab film thicknesses and a small ridge width are directly connected to each other, light propagation loss becomes large due to mismatch in the boundary therebetween. Thus, in the present embodiment, the first intermediate waveguide part 22 a and second intermediate waveguide part 22 b are provided between the first waveguide part 21 a in the RF part 3 a and second waveguide part 21 b in the DC part 3 b so as to reduce the light propagation loss.

The first intermediate waveguide part 22 a is a ridge waveguide connected to the first waveguide part 21 a. The first intermediate waveguide part 22 a has the same slab film thickness (T_(sb1)) as that of the first waveguide part 21 a and has a ridge width W_(1c) (third ridge width) larger than the ridge width of the first waveguide part 21 a. The first intermediate waveguide part 22 a is provided for enlarging the ridge width of the first waveguide part 21 a and has a ridge width enlarged part whose width gradually increases toward the second waveguide part 21 b. Thus, the width W_(1c) (> W₁) of the first intermediate waveguide part 22 a is larger than the width of the first waveguide part 21 a at the connection position with the second intermediate waveguide part 22 b.

The second intermediate waveguide part 22 b is a ridge waveguide connected to the second waveguide part 21 b. The second intermediate waveguide part 22 b has the same slab film thickness (T_(sb2)) as that of the second waveguide part 21 b and has a ridge width W_(2c) (fourth ridge width) larger than the ridge width of the second waveguide part 21 b. The second intermediate waveguide part 22 b is provided for enlarging the ridge width of the second waveguide part 21 b and has a ridge width enlarged part whose width gradually increases toward the first waveguide part 21 a. Thus, the width W_(2c) (> W₂) of the second intermediate waveguide part 22 b is larger than the width of the second waveguide part 21 b at the connection position with the first intermediate waveguide part 22 a.

The ridge width W_(1c) of the first intermediate waveguide part 22 a is larger than the ridge width W_(2c) of the second intermediate waveguide part 22 b. Thus, the ridge width and slab film thickness of the optical waveguide discontinuously vary at the boundary between the first intermediate waveguide part 22 a and second intermediate waveguide part 22 b. When the first waveguide part 21 a in the RF part 3 a and second waveguide part 21 b in the DC part 3 b which are different in the slab film thickness are connected to each other, connection loss increases at the boundary therebetween due to a difference in the spot size of light propagating through the optical waveguide. However, when the ridge width W_(1c) of the optical waveguide on the side close to the RF part 3 a having a relatively smaller slab film thickness is enlarged, and the ridge width W_(2c) of the optical waveguide on the side close to the DC part 3 b having a relatively larger slab film thickness is reduced, it is possible to equalize the light spot size at the boundary between the first waveguide part 21 a and second waveguide part 21 b to thereby reduce connection loss.

The ridge width W_(1c) of the first intermediate waveguide part 22 a is preferably larger than the ridge width W_(2c) of the second intermediate waveguide part 22 b and smaller than twice the ridge width W_(2c) of the second intermediate waveguide part 22 b (W_(2c) < W_(1c) < 2 × W_(2c)) . This can prevent an increase in connection loss due to excessively large ridge width W_(1c) of the first intermediate waveguide part 22 a.

Typically, an optical waveguide having a varying ridge width is easily formed, and a single pattering process allows formation of a taper-shaped optical waveguide having a continuously varying ridge width, so that it is originally unnecessary to form the optical waveguide having a discontinuously varying ridge width. However, formation of the optical waveguide having different slab film thicknesses involves two patterning processes, and it is not easy to align two optical waveguides accurately. Thus, in the present embodiment, the optical waveguides having different slab film thicknesses and provided on the RF part 3 a side and DC part 3 b side, respectively, are processed separately and connected to each other with the ridge widths thereof in the vicinity of a connection part therebetween enlarged. This can reduce connection loss. Thus, the ridge waveguide having discontinuous ridge width and slab film thickness can be obtained.

FIG. 5 is a schematic plan view illustrating a case where an axis misalignment occurs between the optical waveguide on the RF part 3 a side and optical waveguide on the DC part 3 b side.

The waveguide pattern (first waveguide part 21 a and first intermediate waveguide part 22 a) on the RF part 3 a side and waveguide pattern (second waveguide part 21 b and second intermediate waveguide part 22 b) on the DC part 3 b side are processed separately so as to make the slab film thicknesses of the ridge waveguides different therebetween. In this case, as illustrated in FIG. 5 , a misalignment (axis misalignment) of the waveguide pattern may occur. However, in the present embodiment, the ridge widths of the first and second intermediate waveguide parts 22 a and 22 b are enlarged, so that it is possible to prevent a reduction in the sectional area of the waveguide due to the axis misalignment to thereby reduce propagation loss of light due to a processing variation of the waveguide pattern.

As described above, in the optical modulation element 1 according to the present embodiment, the slab film thickness T_(sb2) of the optical waveguide in the DC part 3 b is larger than the slab film thickness T_(sb1) of the optical waveguide in the RF part 3 a, so that it is possible to reduce DC drift while achieving a reduction in driving voltage in the RF part 3 a.

Further, in the optical modulation element 1 according to the present embodiment, in order to connect the optical waveguide (first waveguide part 21 a) in the RF part 3 a and optical waveguide (second waveguide part 21 b) in the DC part 3 b having different slab film thicknesses, the first and second intermediate waveguide parts 22 a and 22 b each having a varying ridge width are provided in the intermediate part 3 c between the RF part 3 a and DC part 3 b, and the ridge width W_(1c) of the first intermediate waveguide part 22 a on the RF part 3 a side is made larger than the ridge width W_(2c) of the second intermediate waveguide part 22 b on the DC part 3 b side, so that it is possible to equalize the spot size of light propagating through the first waveguide part 21 a and spot size of light propagating through the second waveguide part 21 b to thereby reduce propagation loss.

FIG. 6 shows an optical modulation element according to a second embodiment of the present invention, and includes a schematic plan view and schematic cross-sectional views illustrating a waveguide structure around the intermediate part 3 c between the RF part 3 a and DC part 3 b. FIG. 7 is a schematic perspective view three-dimensionally illustrating the waveguide structure in the intermediate part 3 c illustrated in FIG. 6 .

As illustrated in FIGS. 6 and 7 , the optical modulation element 1 according to the present embodiment is featured in that a third intermediate waveguide part 22 c is provided between the first and second intermediate waveguide parts 22 a and 22 b. The third intermediate waveguide part 22 c has: a ridge part 11 r of a two-tier structure composed of a ridge lower part having the same ridge width (W_(1c)) as that of the first intermediate waveguide part 22 a and a ridge upper part having the same ridge width (W_(2c)) as that of the second intermediate waveguide part 22 b; and a slab part 11 s formed on both sides of the two-tiered ridge part 11 r. A slab film thickness T_(sb3) of the third intermediate waveguide part 22 c is smaller than the slab film thickness T_(sb1) of the first intermediate waveguide part 22 a. Although the third intermediate waveguide part 22 c is a ridge waveguide having such a thin slab part 11 s in the present embodiment, the slab part 11 s of the third intermediate waveguide part 22 c may be completely eliminated.

A length Lc of the third intermediate waveguide part 22 c is preferably 3 µm or less. This is because an excessively large length of the third intermediate waveguide part 22 c increases propagation loss.

FIGS. 8A to 8C are views for explaining a formation method for a waveguide structure having the third intermediate waveguide part 22 c illustrated in FIGS. 6 and 7 .

As illustrated in FIGS. 8A to 8C, the waveguide structure having the third intermediate waveguide part 22 c can be formed by partially overlapping a processing area for forming the waveguide pattern in the RF part 3 a and a processing area for forming the waveguide pattern in the DC part 3 b in the vicinity of the boundary therebetween.

Specifically, as illustrated in FIG. 8A, substantially the entire surface of a formation area 25 b for the DC part 3 b is covered with a mask 30 b, and then an electrooptic material film in a formation area 25 a for the RF part 3 a is processed by milling or the like to form the first waveguide part 21 a and first intermediate waveguide part 22 a constituting a waveguide pattern in the RF part 3 a. At this time, an edge E_(b) of the mask 30 b is located at a position that does not reach a boundary B and does not run off to the formation area 25 a for the RF part 3 a, so that a part of the formation area 25 b for the DC part 3 b in the vicinity of the boundary B is subjected to ridge processing. In the processing for the RF part 3 a, a thin slab part can be formed by, for example, increasing a processing time.

Then, as illustrated in FIG. 8B, substantially the entire surface of the formation area 25 a for the RF part 3 a is covered with a mask 30 a, and then an electrooptic material film in the formation area 25 b for the DC part 3 b is processed by milling or the like to form the second waveguide part 21 b and second intermediate waveguide part 22 b constituting a waveform pattern in the DC part 3 b. At this time, an edge E_(a) of the mask 30 a is located at a position that does not reach the boundary B and does not run off to the formation area 25 b for the DC part 3 b, so that a part of the formation area 25 a for the RF part 3 a in the vicinity of the boundary B is subjected to ridge processing. In the processing for the DC part 3 b, a thick slab part can be formed by, for example, reducing a processing time.

As described above, the ridge processing is doubly applied to the vicinity of the boundary B between the formation area 25 a for the RF part 3 a and formation area 25 b for the DC part 3 b, with the result that the third intermediate waveguide part 22 c having a different cross-sectional shape from those of the first and second intermediate waveguide parts 22 a and 22 b is formed as illustrated in FIG. 8C. Although the RF part 3 a and DC part 3 b are processed in this order in the example illustrated in FIG. 8 , the same result can be obtained even when the processing order is reversed.

According to the present embodiment, the same effects as those in the first embodiment can be obtained. That is, the ridge width W_(1c) of the first intermediate waveguide part 22 a on the RF part 3 a side is made larger than the ridge width W_(2c) of the second intermediate waveguide part 22 b on the DC part 3 b side, so that it is possible to equalize the spot size of light propagating through the first waveguide part 21 a and spot size of light propagating through the second waveguide part 21 b to thereby reduce propagation loss.

FIG. 9 shows an optical modulation element according to a third embodiment of the present invention, and includes a schematic plan view and schematic cross-sectional views illustrating a waveguide structure around the intermediate part 3 c between the RF part 3 a and DC part 3 b. FIG. 10 is a schematic perspective view three-dimensionally illustrating the waveguide structure in the intermediate part 3 c illustrated in FIG. 9 .

As illustrated in FIGS. 9 and 10 , the optical modulation element 1 according to the present embodiment is featured in that a third intermediate waveguide part 22 c is provided between the first and second intermediate waveguide parts 22 a and 22 b and that the third intermediate waveguide part 22 c is composed of an unprocessed waveguide layer 11 having no ridge. That is, the third intermediate waveguide part 22 c is an area not having a waveguide shape. Other configurations are the same as those of the second embodiment.

FIGS. 11A to 11B are views for explaining a formation method for a waveguide structure having the third intermediate waveguide part 22 c illustrated in FIGS. 9 and 10 .

As illustrated in FIGS. 11A to 11C, the waveguide structure having the third intermediate waveguide part 22 c can be formed by making a processing area for forming the waveguide pattern in the RF part 3 a and a processing area for forming the waveguide pattern in the DC part 3 b not overlapping each other in the vicinity of the boundary therebetween.

Specifically, as illustrated in FIG. 11A, substantially the entire surface of a formation area 25 b for the DC part 3 b is covered with a mask 30 b, and then an electrooptic material film in a formation area 25 a for the RF part 3 a is processed by milling or the like to form the first waveguide part 21 a and first intermediate waveguide part 22 a constituting a waveguide pattern in the RF part 3 a. At this time, an edge E_(b) of the mask 30 b is positioned beyond a boundary B, i.e., runs off to the formation area 25 a for the RF part 3 a, so that a part of the formation area 25 b for the DC part 3 b in the vicinity of the boundary B is not subjected to ridge processing. In the processing for the RF part 3 a, a thin slab part can be formed by, for example, increasing a processing time.

Then, as illustrated in FIG. 11B, substantially the entire surface of the formation area 25 a for the RF part 3 a is covered with a mask 30 a, and then an electrooptic material film in the formation area 25 b for the DC part 3 b is processed by milling or the like to form the second waveguide part 21 b and second intermediate waveguide part 22 b constituting a waveform pattern in the DC part 3 b. At this time, an edge E_(a) of the mask 30 a is located beyond the boundary B, i.e., runs off to the formation area 25 b for the DC part 3 b, so that a part of the formation area 25 a for the RF part 3 a in the vicinity of the boundary B is not subjected to ridge processing. In the processing for the DC part 3 b, a thick slab part can be formed by, for example, reducing a processing time.

As described above, the unprocessed area is formed in the vicinity of the boundary between the formation area 25 a for the RF part 3 a and formation area 25 b for the DC part 3 b, with the result that the third intermediate waveguide part 22 c having a different cross-sectional shape from those of the first and second intermediate waveguide parts 22 a and 22 b is formed as illustrated in FIG. 11C.

According to the present embodiment, the same effects as those in the first embodiment can be obtained. That is, the ridge width W_(1c) of the first intermediate waveguide part 22 a on the RF part 3 a side is made larger than the ridge width W_(2c) of the second intermediate waveguide part 22 b on the DC part 3 b side, so that it is possible to equalize the spot size of light propagating through the first waveguide part 21 a and spot size of light propagating through the second waveguide part 21 b to thereby reduce propagation loss.

FIGS. 12A and 12B are schematic cross-sectional views of an optical modulation element 1 according to a fourth embodiment of the present invention. FIG. 12A is a cross-sectional view of the RF part 3 a, and FIG. 12B is a cross-sectional view of the DC part 3 b. FIG. 13 is a schematic plan view illustrating a waveguide structure around the intermediate part 3 c between the RF part 3 a and DC part 3 b.

As illustrated in FIGS. 12A, 12B, and 13 , the optical modulation element 1 according to the present embodiment is featured in that the width W₁ of the first and second waveguides 2 a and 2 b (first waveguide part 21 a) in the RF part 3 a and the width W₂ of the first and second waveguides 2 a and 2 b (second waveguide part 21 b) in the DC part 3 b differ from each other. In the present embodiment, the width W₂ of the first and second waveguides 2 a and 2 b (second waveguide part 21 b) in the DC part 3 b is larger than the width W₁ of the first and second waveguides 2 a and 2 b (first waveguide part 21 a) in the RF part 3 a. Other configurations are the same as those of the first embodiment. According to the optical modulation element 1 of the present embodiment, it is possible not only to obtain the same effects as those in the first embodiment but also to enhance the DC drift reduction effect.

FIGS. 14A and 14B are schematic cross-sectional views of an optical modulation element 1 according to a fifth embodiment of the present invention. FIG. 14A is a cross-sectional view of the RF part 3 a, and FIG. 14B is a cross-sectional view of the DC part 3 b.

As illustrated in FIGS. 14A and 14B, the optical modulation element 1 according to the present embodiment is featured in that the protective layer 12 is omitted in the DC part 3 b. That is, the DC part 3 b of the optical modulation element 1 has a multilayer structure including the substrate 10, waveguide layer 11, buffer layer 13, and electrode layer 14, which are laminated in this order. The buffer layer 13 is formed over the entire surface of the waveguide layer 11 so as to cover not only the upper surface of the ridge part 11 r but also the side surfaces thereof. Accordingly, the upper surface of the slab part 11 s on both sides of the ridge part 11 r is also covered with the buffer layer 13.

When the protective layer 12 is omitted in the DC part 3 b, the buffer layer 13 in the DC part 3 b is preferably made of a dielectric material different from that of the buffer layer 13 in the RF part 3 a and is preferably made of the same dielectric material as that of the protective layer 12 in the RF part 3 a. That is, the buffer layer 13 in the DC part 3 b is preferably formed using the same dielectric material as that of the protective layer 12 in the RF part 3 a. The configuration in which the protective layer 12 is omitted in the DC part 3 b is equivalent to a configuration in which the protective layer 12 and buffer layer 13 in the DC part 3 b illustrated in FIG. 2B are formed using the same material.

While the preferred embodiments of the present invention have been described, the present invention is not limited to the above embodiments, and various modifications may be made within the scope of the present invention, and all such modifications are included in the present invention.

For example, in the above respective embodiments, an optical modulation element of a dual drive type in which a pair of signal electrodes are provided for a Mach-Zehnder optical waveguide having a pair of optical waveguides has been described; however, the present invention is not limited to optical modulation elements having such a configuration but may be applied to various optical modulation elements having the RF part 3 a and DC part 3 b.

Further, in the above respective embodiments, the optical modulation element has a pair of optical waveguides each formed of the lithium niobate film epitaxial grown on the substrate 10; however, the present invention is not limited to such a structure, but the optical waveguides may be formed of an electrooptic material such as barium titanate or lead zirconium titanate. However, the optical waveguide formed of the lithium niobate film can be reduced in width to achieve a reduction in driving voltage, whereas a problem of DC drift is conspicuous, and thus the effects of the present invention are great. Further, as the waveguide layer 11, a semiconductor material, a polymer material, or the like having electrooptic effect may be used.

EXAMPLES Evaluation of Slab Film Thickness in RF Part 3 a

The influence of the slab film thickness T_(sb1) of the ridge waveguide in the RF part 3 a on the electric field efficiency VπL was evaluated by simulation. The thickness of the waveguide layer 11 formed of a lithium niobate film was set to 1.5 µm, and the electric field efficiency VπL was calculated while varying the slab film thickness T_(sb1). As a result, as illustrated in FIG. 15 , the VπL became minimum when the slab film thickness T_(sb1) of the ridge waveguide was about 0.3 µm. Further, when the slab film thickness T_(sb2) was 0.6 µm, the VπL fell within a preferable range of 2.2 Vcm or less.

Evaluation of Slab Film Thickness in DC Part

The influence of the slab film thickness T_(sb2) of the ridge waveguide in the DC part 3 b on DC drift was evaluated. In this evaluation test, the amount of DC drift was measured while applying a constant bias voltage to a bias electrode under a temperature of 80° C., and a time (lifetime) required from the start of the evaluation test until the DC drift amount exceeded 50% was measured. The thickness of the waveguide layer 11 formed of a lithium niobate film was set to 1.5 µm. As a result, the lifetime was about one hour when the slab film thickness T_(sb2) is 0.3 µm and about 84 hours when the slab film thickness T_(sb2) is 0.6 µm. On the other hand, when the slab film thickness Tsb2 was 1.1 µm, the lifetime was as satisfactory as more than 1000 hours.

Evaluation of Ridge Width of First and Second Intermediate Waveguide Parts

For the waveguide structure illustrated in FIGS. 3 and 4 , a variation in connection loss (dB) was evaluated by simulation while varying the ridge width W_(1c) of the first intermediate waveguide part 22 a. The thickness of the waveguide layer 11 formed of a lithium niobate film was set to 1.5 µm, the ridge width W_(2c) of the second intermediate waveguide part 22 b was set to 2.5 µm, the slab film thickness T_(sb1) of the first intermediate waveguide part 22 a was set to 0.4 µm, and the slab film thickness T_(sb2) of the second intermediate waveguide part 22 b was set to 1.1 µm. As a result, as illustrated in FIG. 16 , connection loss became minimum in the range of the ridge width W_(1c) of 3 µm to 4.5 µm. Thus, a reduction in connection loss of a ridge wave guide having different slab film thicknesses was confirmed.

Evaluation of Misalignment of Optical Waveguide

The influence of a magnitude ΔW of axis misalignment of the optical guide illustrated in FIGS. 11A to 11C on connection loss was evaluated by simulation. The thickness of the waveguide layer 11 formed of a lithium niobate film was set to 1.5 µm, and five combinations of the ridge widths (W_(1c), W_(2c)) of the first and second intermediate waveguide parts 22 a and 22 b were set as follows: (1.2 µm, 2.5 µm); (2.5 µm, 2.5 µm); (3.8 µm, 2.5 µm); (4.2 µm, 3 µm); and (5.2 µm, 4 µm). Further, the slab film thickness T_(sb1) of the first intermediate waveguide part 22 a was set to 0.4 µm, and the slab film thickness T_(sb2) of the second intermediate waveguide part 22 b was set to 1.1 µm.

As a result, as illustrated in FIG. 17 , in the absence of the axis misalignment, connection loss of the optical waveguide tended to increase as the ridge widths W_(1c) and W_(2c) of the respective first and second intermediate waveguide parts 22 a and 22 b reduced. In the presence of the axis misalignment, connection loss increased with an increase in the axis misalignment. Specifically, when the ridge widths (W_(1c), W_(2c)) of the first and second intermediate waveguide parts 22 a and 22 b were (1.2 µm, 2.5 µm), the increasing rate of connection loss with respect to the axis misalignment was highest, and when the ridge widths (W_(1c), W_(2c)) of the first and second intermediate waveguide parts 22 a and 22 b were (5.2 µm, 4 µm), the increasing rate of connection loss with respect to the axis misalignment was lowest. In summary, the smaller the ridge widths W_(1c) and W_(2c) the smaller the ridge widths W_(1c) and W_(2c) of the first and second intermediate waveguide parts 22 a and 22 b, the higher the increasing rate of connection loss.

Evaluation of Third Intermediate Waveguide Part

The influence of the presence of the third intermediate waveguide part 22 c illustrated in FIGS. 6 to 11 on connection loss (dB) was evaluated by simulation. The thickness of the waveguide layer 11 formed of a lithium niobate film was set to 1.5 µm, the slab film thickness T_(sb1) of the first intermediate waveguide part 22 a was set to 0.4 µm, and the slab film thickness T_(sb2) of the second intermediate waveguide part 22 b was set to 1.1 µm. Further, three combinations of the ridge widths (W_(1c), W_(2c)) of the first and second intermediate waveguide parts 22 a and 22 b were set as follows: (3.8 µm, 2.5 µm); (4.2 µm, 3 µm); and (5.2 µm, 4 µm). For each of the above three combinations, a case (FIGS. 6 to 8 ) where the slab film thickness was small (in this simulation, slab film thickness was set to 0) and a case (FIGS. 9 to 11 ) where the waveguide structure is absent were evaluated in combination.

As a result, as illustrated in FIG. 18 , connection loss increased as the length of the third intermediate waveguide part 22 c increased. In particular, the increasing rate of connection loss was higher for the third intermediate waveguide part 22 c illustrated in FIGS. 9 to 11 having no waveguide structure than for the third intermediate waveguide part 22 c illustrated in FIGS. 6 to 8 having no slab part. However, in either structure, connection loss was 0.2 dB or less when the length of the third intermediate waveguide part 22 c was 3 µm or less. It is fully possible to manufacture the third intermediate waveguide part 22 c having a length of 3 µm or less, and the third intermediate waveguide part 22 c having such a length does not cause any practical problems.

REFERENCE SIGNS LIST

-   1: Optical modulation element -   2: Mach-Zehnder optical waveguide -   2 a: First waveguide -   2 b: Second waveguide -   2 c: Demultiplexer -   2 d: Multiplexer -   2 i: Input waveguide -   2 o: Output waveguide -   3 a: RF part -   3 b: DC part -   3 c: Intermediate part -   3 id: DC interaction part -   3 ir: RF interaction part -   4 a: First signal electrode -   4 a ₁: One end of first signal electrode -   4 a ₂: The other end of first signal electrode -   4 b: Second signal electrode -   4 b ₁: One end of first signal electrode -   4 b ₂: The other end of first signal electrode -   5 a: First bias electrode -   5 a ₁: One end of first bias electrode -   5 b: Second bias electrode -   5 b ₁: One end of second bias electrode -   9: Terminal resistor -   9 a: Driver circuit -   9 c: Bias circuit -   10: Substrate -   11: Waveguide layer -   11 r: Ridge part -   11 s: Slab part -   12: Protective layer -   13: Buffer layer -   14: Electrode layer -   21 a: First waveguide part -   21 b: Second waveguide part -   22 a: First intermediate waveguide part -   22 b: Second intermediate waveguide part -   22 c: Third intermediate waveguide part -   30 a: Mask -   30 b: Mask 

1. An optical waveguide element comprising: a substrate; and an optical waveguide formed of a thin film on the substrate and having a ridge part which is a protruding portion and a slab part having a smaller film thickness than the ridge part, wherein the optical waveguide includes: a first waveguide part having a first ridge width and a first slab film thickness; a second waveguide part having a second ridge width and a second slab film thickness; a first intermediate waveguide part connected to the first waveguide part and having a third ridge width and the first slab film thickness; and a second intermediate waveguide part connected to the second waveguide part and having a fourth ridge width and the second slab film thickness, the first waveguide part, first intermediate waveguide part, second intermediate waveguide part, and second waveguide part are arranged in this order, the second slab film thickness is larger than the first slab film thickness, and the third ridge width is larger than the fourth ridge width.
 2. The optical waveguide element according to claim 1, wherein the third ridge width is smaller than twice the fourth ridge width.
 3. The optical waveguide element according to claim 1, wherein the third ridge width is larger than the first ridge width, and the fourth ridge width is larger than the second ridge width.
 4. The optical waveguide element according to claim 1, wherein the second ridge width is larger than the first ridge width.
 5. The optical waveguide element according to claim 1, wherein the optical waveguide further includes a third intermediate waveguide part provided between the first and second intermediate waveguide parts and having a cross-sectional shape different from those of the first and second intermediate waveguide parts.
 6. The optical waveguide element according to claim 5, wherein the third intermediate waveguide part has a third slab film thickness smaller than the first slab film thickness.
 7. The optical waveguide element according to claim 5, wherein the third intermediate waveguide part has no slab part.
 8. The optical waveguide element according to claim 5, wherein the third intermediate waveguide part has no waveguide shape.
 9. The optical waveguide element according to claim 5, wherein a length of the third intermediate waveguide part is 3 µm or less.
 10. The optical waveguide element according to claim 1, wherein the first slab film thickness is less than 0.6 µm, and the second slab film thickness is 0.6 µm or more.
 11. The optical waveguide element according to claim 1, wherein the thin film is a lithium niobate film, and the c-axis of the lithium niobate film is oriented perpendicular to the main surface of the substrate.
 12. An optical modulation element comprising: an optical waveguide element according to claim 1; a signal electrode that applies an RF signal to the first waveguide part; and a bias electrode that applies a DC bias to the second waveguide part, wherein the thin film is made of an electrooptic material. 